In vehicles, there is a hybrid vehicle in which an engine driven by combustion of fuel is directly connected to a motor generator (assist motor) driven by electric power and having a power generating function. This hybrid vehicle may include the engine, the motor generator to drive and assist the engine, and an automatic transmission equipped with a torque converter having a lockup mechanism (i.e., a clutch).
The hybrid vehicle is conventionally provided with the engine, the motor generator (assist motor) to improve fuel efficiency, and the automatic transmission with the torque converter. The lockup mechanism (clutch) is associated with the torque converter so as to improve transmission efficiency of the torque converter (see JP Application Laid-Open No. 2002-147600).
Another example of the hybrid vehicle includes the motor generator (assist motor) as well as the engine, and the motor generator assists the driving torque in a range where the engine load is heavy, whereas the motor generator does not assist the driving torque in a range where the engine load is light, so that the motor is driven frequently in a range where the efficiency of the engine is relatively high. The load range for assisting is expanded toward the lighter side if the amount of battery remaining is enough (see JP Application Laid-Open No. H09-84210).
The automatic transmission with the torque converter is equipped with the lockup mechanism (e.g. clutch) to improve transmission efficiency of the torque converter. The lockup clutch is controlled to lockup in a high-speed range where variation of the engine torque is permissible, and to permit slip in a mid-speed range where the variation of the engine torque is not permissible. Generally, the range for slipping is limited to a low-to-mid load range to maintain durability, since the slipping lockup mechanism produces frictional heating.
In a conventional controller for the hybrid vehicle which controls the assist torque in combination with the automatic transmission with the torque converter in which the lockup mechanism (clutch) can be slipped, assisting by the motor in the slip range increases the heating value of the lockup mechanism in comparison to a case where the motor does not assist, thereby reducing durability of the lockup mechanism. To obviate this problem, the slip range may be narrowed toward the lighter load side. However, this increases fuel consumption.